1. Field of the Invention
The present invention relates to a method of and an apparatus for correcting color patterns in a multicolor printing process, and more particularly, it relates to a method of and an apparatus for correcting color patterns which are overprinted to produce a multicolor print image, thereby to make color mismatching which arise from printing block registration difficulties.
2. Description of the Prior Art
As well known in the art, a multicolor printing process is performed through overprinting several color images on a same printing paper with blocks of different colors.
As shown in FIG. 1A, for example, when a multicolor image consists of a first region 1 which is printed with a first color ink and a second region 2 which is overprinted with the first color ink and a second color ink, a first block for printing the first color ink, and a second block for the second color ink are produced first. The first block has a first print pattern 3, as shown in FIG. 1B, and the second block has a second print pattern 4. Then, the printing process is performed with the first and second blocks on a same printing paper. In this example, each of the first and second print patterns 3 and 4 is formed as a tint pattern which consists of same halftone dots with same halftone area rates, because the first and second tint patterns 3 and 4 have uniform color densities, respectively. Desirable tones of the respective print patterns are achievable through setting the halftone area rates of the first and second print patterns relative to one another.
In the above process, block registration matching, which means that the first and second print patterns 3 and 4 are overprinted on precise positions of a same printing paper, is at extreme importance. If the registration matching is off, jut regions 3a, 3b and 4a of a single color are formed, as shown in FIG. 1C. This deteriorates the quality of the multicolor image. If the first print pattern 3 is of relatively light density and the second print pattern 4 is of relatively deep density, the jut regions 3a and 3b are conspicuous and the other jet region 4a is not so conspicuous.
In general, the registration mismatch problem is prevented through controlling the dimensions of the print patterns in the block making process and by precisely controlling printing positions of respective print patterns in printing process. However, because registering is performed using edge portion of a printing paper as a base position, the registration mismatch is often caused due to expansion or contraction of the printing paper during a first color print and a second color print, if the printing paper is easy to expand or contract. Such registration mismatch can not be prevented through improving of the printing process or the printing machine.
FIG. 2 illustrates another example of a registration mismatch problem. A third print pattern 8 and a fourth print pattern 9 must contact each other, though an unprinted line region 10 is formed between the third and fourth print patterns 8 and 9 due to the registration mismatch problem. The registration mismatch is also caused by expansion or contraction of the printing paper.